This invention relates to a printing blanket and method of reducing corrosion and abrasion of printing blankets and printing blanket cylinders, and more particularly, to a metal-backed printing blanket including a protective polymeric film layer on the outer surface of the metal backing which contacts the surface of the printing cylinder when the blanket is mounted thereon.
One of the most common commercial printing processes is offset lithography. In this printing process, ink is offset from a printing plate to a rubber-surfaced printing blanket mounted on a printing cylinder before being transferred to a substrate, such as paper. The blanket cylinder generally comprises a chrome or nickel-plated steel cylinder having an encircling rubber printing blanket which is releasably mounted to the cylinder.
The printing blanket is generally reinforced with either a fabric backing or a metal backing. Unlike the fabric backing, which exhibits a high degree of stretch, the metal backing avoids stretching of the blanket when it is tensioned on the blanket cylinder, which provides better print quality and eliminates the need for frequent retensioning of the blanket.
However, the use of metal-backed printing blankets has resulted in a greater potential for corrosion and abrasion of the printing blanket as well as the blanket cylinder surface. The use of water, inks and chemicals during the printing process results in an accumulation of moisture between the printing blanket and the cylinder surface. After extended use, this accumulation of moisture can cause corrosion of the metal backing on the printing blanket as well as the surface of the blanket cylinder. In addition, the intimate contact of the metal backing of the printing blanket with the metal surface of a blanket cylinder and the rapid rotation of the two results in a build-up of friction which abrades the surface of the blanket cylinder, also contributing to corrosion.
Many efforts have been made to reduce or eliminate corrosion of printing blanket cylinders. For example, blanket cylinders have been developed which are comprised of corrosion-resistant materials such as stainless steel. More recently, blanket cylinders have had ceramic materials applied to the surfaces thereof in an attempt to reduce or eliminate corrosion. However, the use of such materials alone has not been found to fully eliminate corrosion and abrasion problems.
Printing blankets have also been modified in an attempt to reduce corrosion. For example, metals such as zinc have been incorporated into the backing layers of printing blankets. Such metals act as sacrificial metals which are more chemically active than the metal contained in the cylinders. See U.S. Pat. No. 5,366,784 and EP Patent No. 0593593.
Other attempts have been made to apply a rubber coating on the fabric base of a printing blanket to restrict the wicking of moisture beneath the blanket and/or to eliminate abrasion during contact of the blanket with the cylinder surface. See, for example, Spöring, U.S. Pat. No. 4,471,011, and Pinkston et al., U.S. Pat. No. 5,006,400. However, such methods have been used in conjunction with fabric backings. It would be desirable to be able to provide corrosion and abrasion resistance while maintaining the benefits provided by the use of metal-backed blankets.
Accordingly, there is still a need in the art for a base or backing for printing blankets used in offset printing operations which provides the desired printing properties while reducing corrosion and/or abrasion of the blanket and/or blanket cylinder.